The present invention relates to a system for controlling a steering motor for turning steerable vehicle wheels. In particular, the present invention relates to a electro-hydraulic vehicle steering system which does not have a mechanical connection between the vehicle steering wheel and the steerable vehicle wheels.
Electro-hydraulic steering systems which do not have a mechanical connection between the vehicle steering wheel and the steerable vehicle wheels are known. These systems typically include a sensor for sensing rotation of the vehicle steering wheel. A hydraulic actuator is actuated by a control device in response to a signal provided by the sensor for sensing rotation of the vehicle steering wheel. The hydraulic actuator, when actuated turns the steerable vehicle wheel.
The sensor for sensing rotation of the vehicle steering wheel is often a single rotary, electrical potentiometer. The potentiometer is mounted such that the stator of the potentiometer is fixedly attached to a support for the vehicle steering wheel and the rotor of the potentiometer is rotated upon rotation of the vehicle steering wheel. The potentiometer provides an output voltage signal which is linearly proportional to the rotational position of the vehicle steering wheel for a substantial portion of rotation of the vehicle steering wheel. However, the output voltage signal of the potentiometer ceases to be linearly proportional to the rotational position of the steering wheel in regions encompassing the maximum and minimum for the output voltage signal of the potentiometer. Also, the output voltage signal of the potentiometer abruptly changes as the vehicle steering wheel is rotated through a portion of rotation corresponding to a transition or discontinuity zone of the potentiometer. Thus, steering may be either not fully responsive to rotation of the vehicle steering wheel or "jerky."
Also, if the potentiometer fails to provide an output voltage signal, the system becomes inoperative. Such a situation could occur if damage occurs to either the potentiometer or the electrical lead wires for the potentiometer. The vehicle would be rendered unsteerable until the problem is corrected. In such a situation, this could have the unfortunate consequence of stranding the vehicle at a location remote from service facilities.